Hydraulic fluids



United States Patent Ofllice 3,305,487 Patented Feb. 21, 1967 3,305,487 HYDRAULIC FLUIDS Norman Ernest Frederick Hitchcock, Robert Alan Cameron Ker, and Brian Thomas Scoltock, London, England, assignors to Castro] Limited, London, England, a company of Great Britain N Drawing. Filed Sept. 3, 1963, Ser. No. 306,284 Claims priority, application Great Britain, Sept. 4, 1962, 33,909/ 62 6 Claims. (Cl. 25278) The invention relates to compositions suitable for use as aviation hydraulic fluids, especially those hydraulic fluids which are designated fire resistant, according to the usual understanding of that term in the art.

We have surprisingly discovered that certain brominated ethyl benzenes may be diluted with a proportion of a mineral oil of lubricating viscosity without losing their highly desirable fire resistant properties; indeed we have discovered that such compositions need only contain a minimum of, for example, 50% of dibromoethyl benzene to have satisfactory fire resistant characteristics.

Aviation hydraulic fluids, i.e., fluids used in the hydraulic systems of aircraft, are required to remain freeflowing at very low temperatures and to have high viscosity indices, good shear stability and high bulk modulus. They must also have good oxidation resistance, be non-corrosive towards the metals with which they come into contact at operating temperatures and, preferably, be compatible with other fluids likely to be used in the same system.

Suitable hydraulic fluids may be based on selected mineral oils of low viscosity and low pour point in accordance with British Specification DTD-SSS or United States Specification MIL-H-5606, but these fluids, although otherwise satisfactory, are not fire resistant.

Other widely used aviation hydraulic fluids are based on certain phosphate esters but these fluids, although having reasonably :good fire resistance, exhibit rather poor resistance to oxidation and are corrosive towards certain metals, especially brass and cadmium-plated steel, at elevated temperatures. In order to use these fluids the system has to be designed to keep the fluid cool. Yet another type of fire resistant aviation hydraulic fluid is based upon polysiloxanes containing chlorinated phenyl su-bstituents. These fluids have good viscosity-temperature characteristics and oxidation resistance but suffer from the disadvantage of having poor lubricity and a low bulk modulus. Neither of these two types of synthetic hydraulic fluid are fully compatible with fluids based on mineral oils.

It is an object of the present invention to provide novel fire-resistant aviation hydraulic fluids having good viscosity-temperature characteristics, good oxidation resistance and lubricity and which are at the same time compatible and miscible with fluids based on mineral oils complying with DTD-585 or M-IL-H-S606 specification.

The present invention therefore provides a novel composition of matter suitable for use as an aviation hydraulic fluid comprising '(a) a liquid mixture of isomeric brominated ethyl benzenes having the general formula:

where Z is from 2 to 3, (b) a mineral oil or mixture of mineral oils having a flash point of at least 200 F. and a pour point of less than -F., and (c) from 2% to 20% by weight of a polymeric thickening agent such that the composition has a viscosity of at least 6 cs. at F. and not more than 600 cs. at -40 F., wherein the composition contains the ingredient (a) in an amount to provide from 30 to 45% by weight of bromine.

Preferably the polymeric thickening agent is a polyisobutylene or an alkyl polymethacrylate of relatively low molecular weight and good shear stability. Preferably the composition has a viscosity of at least 8 cs. at 130 F. and not more than 500 cs. at -40 F.

It is preferred that, in addition to the three components (a), (b) and (c), the fluids of the present invention should contain at least one antioxidant (d) which may be either a secondary aromatic amine or a so-called hindered alkyl phenol. Such phenols contain at least one tertiary alkyl group in the ortho position to the hy-droxy group. Suitable examples of additive (d) are phenyl-ot-naphthylamine, phenyl-[i-naphthylamine, 2,6-di-tertiary butyl-pcresol and methylene bis (2,6-di-tertiary butyl) phenol. It is desirable to have present a mixture of antioxidants one of which is a secondary aromatic amine, the other being a hindered phenol. The antioxidants may be present in the composition in amounts of from 0.1 to 2.0%

'by weight.

It is desirable also to have present in the composition one or more lubricity additives, tricresyl phosphate being especially suitable, and such additive may be present in the composition in amounts of 0.3 to 1.5% by weight.

Metal deactivators such as benzotriazole, may also ad- 'vantageously be present. The metal deactivator will, of course, be employed in an amount sufficient to provide the desired degree of deactivation; benzotriazole may, for example, be employed in amounts of from 0.005 to 0.05% by 'Weight.

It has been found that for the fluid to have adequate fire resistance at least 30% of bromine must be present. Since it is desirable that the specific gravity of the fluid should be as low as possible and certainly not more than 1.5, not more than 45 of bromine should be present. For economic reasons it is desirable that the bromine content of the fluid should be kept as low as possible consistent with obtaining the desired degree of fire resistance. When a mixture of isomeric dibromo ethyl benzenes are used, a minimum of about 50% by weight will be required to provide the necessary amount of bromine in the fluid. When using a more highly brominated product such' as Dow Gauge Fluid R-ZOO, which contains an average of 2.9 bromine atoms per benzene ring, a minimum of about 45 by weight may be used.

In addition to the material known as Dow Gauge Fluid R-200, which is a mixture of diand tri-brornoethyl benzenes, there are available commercially technical grades of di-bromoethyl benzene" consisting of a mixture of isomeric mono-bromo, dibromo and tribromoethyl 4 Example III benzenes of probable structures: Percent Br Dibromoethyl benzene 60.49 Mineral oil A 15 C2H5 --C2H5 C2H5 5 Mineral Oil B 5 Viscoplex H-7 9 Br Br Tri-m-cresyl phosphate 0.5 Br B! Benzotriazole 0.01 Such that the average number of bromine atQmS in the This composition was found to have the following propsubstituted aromatic nucleus of the molecule 1s approxi- 10 erties: mately two. Also available are distilled grades of dio bromoethyl' benzene consisting essentially of mixed isovlscoslty at 130 a 9 meric dibromoethyl benzenes. Vlscoslty 500 Mixtures of isomeric brominated alkyl benzenes, suita- 240 ble for use in the compositions of the present invention, 1 Pour Pomt may be prepared by [nominating monoethyl benzene at The composmon, Example III, represents an av1 at1on room temperature in the presence of iron filings as hydraulic fluid which has been found to be fire resistant scribed in United States patent specification No. 2,257,- when tested b the t disctlbed Natlonal cotfll 903. Board Provisional Specificat on Fire Res1stant Hydraulic The polymers which are employed as component (c) Fluld U56 Underground. in the compositions of the present invention are prefer- To demonstrate the utlllty 0f t p sition of Examably chosen to be of such molecular weight as to provide ple h blenfl was tested ctlmparfson Wlth two fluids having shear stabilities meeting the requirements of Pther avallable arcraf't hydrauhc fluids Wtth the follow Specifications DTD-585 and MIL-H-5606. They may mg results- Low Temp. Viscosity Shear Miscibility Oxidation Corrosion Fire Resistance Material Props. Index Stability with gliineral Resistance Resistance Spray Test Composition otExampleS Good High Good Yes Good Good Pass. Commercial Material A Fair Mcdiu Poor N Poor D0. Commercial Material B do Good Do.

be polyisobutylenes such as materials commercial-1y available under the trade names Indopol, P olyvis and Oronite PB, having molecular weights in the range 1000-3000 or alkyl polymethacrylates of good shear stability such as materials commercially available under the trade names viscoplex H-7 or Acryloid (Plexol) HF-855. The alkyl groups of the alkyl polymeth'acrylates may contain from 4 to 10* carbon atoms and the average molecular weight of these materials may be from 2,000 to 20,000 and is preferably from 5,000 to 15,000. When polyisobutylenes are used in the compositions of the present invention, they will normally be used in amounts ranging from 10-20% by weight on the weight of the composition, whereas when polymethacrylates are used, as little as 240% by weight on the weight of the composition will normally be necessary.

As specific examples of the present invention the following compositions are set forth, percentages being by weight.

Example I Percent Dibromoethyl benzene (technical grade) 61 Mineral oil A 26 Indopol H-1900 13 Mineral oil A was a spindle oil of viscosity about 40 seconds Redwood I at 140 F.

Example 11 Percent Dow Gauge Fluid R-200" Mineral oilA 35 Mineral oil B 7 Acryloid HF-SSS 8 Mineral oil B was a gas oil fraction of viscosity about 3-4 cs. at 100 F.

The physical properties of Examples I and II were generally similar to those of Examples VIII and VII, respectively.

Commercial Material A was of the phosphate ester type and Commercial Material B of the chlorophenyl polysiloxane type.

Blends according to the present invention are miscible with mineral oil blends and especially with aviation hydraulic oil to Specification DTD-585. Thus, when the oil in an aircraft is changed from DTD-585 to a fire resistant fluid, it would not be necessary to disassemble and clean and flush the whole system free of mineral oil if the replacement oil were miscible with any remaining traces of the original; this represents a distinct advantage of these fluids.

Further examples of compositions prepared in accordance with the present invention are listed in Table I, the physical properties of these fluids being given in Table II together with comparative data on a mineral oil-based non fire-resistant fluid conforming to DTD-585 specification.

TABLE I.COMPOSITIONS OF EXAMPLES IV-VIII ExamplcNo VI VII VIII Percent by weight of- Dibromoethyl benzene (technical) Dow Gauge Fluid R-200 Mineral Oil A. Mineral Oil B... Viscoplex H-7 Indopol H1900 All these five blends contained the following additive:

By wt. on the wt. of the composition, percent Benzotriazole 0.01

TABLE II,PHYSICAL TEST DATA Typical Fluid to Ex. IV Ex. V EX. VI Ex. VII Ex. VIII DTD-585 Specification DTD-585 Requirements Spccn Viscosity, cs.:

At 130 F 10. 3 8. 6 8.3 9. 8. 4 6. 8 10.0 min. At -40 F 490 480 440 425 540 450 500 max. Pour point, F 75 -80 75 80 80 80 75. Flash point, F 205 250 265 290 245 245 200 min. Neutralisation N0 0. 09 0. 06 0. 07 0. 09 0. 0. 03 0.2 max. Low Tei ngperature Stability (72 hrs. at Pass Pass Pass Pass Pass Pass Clouding to be no worse than standard.

-65" Tackincss Pass Pass Pass Pass Pass Pass Not greater than reference fluid. Shear Stability, Percent 25 22 Nrzggnore than given by reference fluid Copper strip test (4 hours at 120 0.)... Pass Pass Pass Pass Pass Pass No grey, brown or black stain. Bull: l\)/lodulus p.s.i. at 77 F. (05,000 230,000 200, 000

p.s.i.

Tests were carried out by the methods specified in DTD585 specification.

Commercial Fluid A had a bulk modulus of 208,000 p.s.i. at 77 F. (0-500 p.s.i.) and Commercial Fluid B a bulk modulus of only 170,000 p.s.i, at 77 F. (0-5000 p.s.i.).

It will be seen from Table II that the compositions of the present invention were essentially similar in properties to a fluid complying with DTD-5 85 specification and known to be satisfactory in service in aircraft.

In order to determine the fire resistance properties of the fluids of the present invention, tests were carried out by two diiferent procedures known as the wick test and the spray ignition test.

In the spray ignition test, the fluid, at a pressure of 1000 psi. and at a temperature of 85 C., is sprayed from an oil burning nozzle. An oxyacetylene flame from a welding blowpipe is repeatedly applied and withdrawn from diflerent positions in the spray and the maximum duration of burning after withdrawal of the igniting flame is recorded (30 seconds maximum).

In the wick test an asbestos wick is soaked in the fluid and then suspended half immersed in the fluid. A non-aerated acetylene flame is applied to the edge of the wick for various application times and the duration of any resulting flame is recorded. Five tests are done for each application time and the average flame duration calculated. The largest average is taken to be the maximum burning time (60 secs. maximum).

Test results on various blends are listed in Table HI.

TABLE III.FIRE RESISTANCE TESTS 1 Maximum burning time, seconds. 2 Bums continuously. 3 N0 flame.

The compositions of the present invention therefore exhibited satisfactory fire-resistance but Example IV, which contained only 50% of dibromoethyl benzene and contained only 29% of bromine, was borderline, in that it passed the spray ignition test but failed the wick test.

The corrosion properties of Example III and Cornmercial Fluid A were compared by means of a simple test in which single metal specimens were immersed in the fluid contained in a 2-oz. screw-capped jar and heated in an oven for 2 months at C. The results are recorded in Table IV.

TABLE IV.CORROSION TESTS AT 120 0.

[Weight changes (mg/sq. cm.) after 2 months] Commercial Example III Fluid A Nil 0. 10

Brass -75. 0 +0.10 Nickel plated steel. 0. 1O 0. 10 Cadmium-plated steel -35. 0 -0. 10

The superiority of the fluid of the present invention over the commercial fluid was thus manifest.

In Table V are listed the results of oxidation/ corrosion tests carried out in accordance with DTD-585 specification. The test was carried out at 120 C. with an air blowing rate of 5 litres per hour. In this test the oils (100 ml.) were placed in large test-tubes (30 cm. long, 6.5 cm. outside diameter) fitted with reflux condensers, the metal specimens being suspended in the oils in the form of a pentagon.

It will be seen that all the examples in accordance with the present invention had very satisfactory oxidation and corrosion characteristics.

TABLE V.DTD-585 OXIDATION/CORROSION TESTS Typical Examples fluid to DTD-585 DTD Specification 585 IV V VI VII VIII Requirement Spccn.

N eutralisation N0. increase 0. 09 0. 17 0. 11 0. 14 0. 19 0. 13 0.2 max. Viscosity change (cs. at F.), pcrcent +2. 8 +3. 5 +4. 5 2. 2 Nil +6. 5 -5 to +20. Weight change of metal specimens (mgj s cm.

q Nil Nil Nil 330.2 max. Cadmium plated steel. +0. 04 Nil Nil 510.2 max. C Nil Nil Nil 3:0.6 max. Aluminium alloy Nil Nil Nil 5:02 max. Magnesium alloy Ni Nil Nil +0.2 max.

7 We claim: 1. A novel composition of matter suitable for use as an aviation hydraulic fluid consisting essentially of (a) a liquid mixture of isomeric brominated ethyl benzenes having the general formula:

where z is from 2 to 3,

(b) a mineral oil having a flash point of at least 200 F.

and a pour point of less than -75 F., and

(c) from 2% to 20% by weight of a polymeric thickening agent selected from the group consisting of polyisobutylenes having average molecular weights of from 1000 to 3000, and alkyl polymethacrylates having average molecular weights of from 2000 to 20,000; and from 4 to 10 carbon atoms in each alkyl group such that the composition has a viscosity of at least 6 cs. at 130 F. and not more than 600 cs. at -40 F., wherein the composition contains the ingredient (a) in an amount to provide from 30 to 45% by weight of bromine.

2. A composition as claimed in claim 1 wherein the composition has a viscosity of at least 8 cs. at 130 F. and not more than 500 cs. at -40 F.

3. A composition as claimed in claim 1 wherein the composition contains from 5 to by weight of said alkyl polymethacrylate, the alkyl groups of which contain from 6 to 10 carbon atoms, from 0.3 to 1.0% by weight of tricresyl phosphate and from 0.007 to 0.015% by weight of benzotriazole.

4. A composition as claimed in claim 3 wherein the composition also contains from 0.2 to 1.0% by weight of both a secondary aromatic amine selected from the group consisting of phenyl-a-naphthylamine and phenyl- 5 fi-naphthylamine and a hindered alkyl phenol selected from the group consisting of 2,6-di-tertiary butyl-p-cresol and methylene bis(2,6-di-tertiary butyl) phenol.

5. A composition as claimed in claim 4 wherein ingredient (a) is present in an amount of from 55 to 65% 10 by weight and ingredient (b) is present in an amount of from 25 to 43% by weight.

6. A composition as claimed in claim 1 wherein the composition also contains at least one antioxidant selected from the group consisting of phenyl-a-napthylamine, phenyl-fi-naphthylamine, 2,6-di-tertiary butyl-pcresol and methylene bis(2,6-ditertiary butyl) phenol.

References Cited by the Examiner UNITED STATES PATENTS 2,257,903 10/1941 Dreisbach 252-78 X 2,528,348 10/1950 Denison et al 252 7s X 2,549,270 4/1951 Watson 252 7s 3,050,465 8/1962 Francis 252-77 X FOREIGN PATENTS 823,295 11/1959 Great Britain.

LEON D. ROSDOL, Primary Examiner.

ALBERT T. MEYERS, SAMUEL H. BLECH,

Examiners.

R. D. LOVERING, S. D. SCHWARTZ,

Assistant Examiners. 

1. A NOVEL COMPOSITION OF MATTER SUITABLE FOR USE AS AN AVIATION HYDRAULIC FLUID CONSISTING ESSENTIALLY OF (A) A LIQUID MIXTURE OF ISOMERIC BROMINATED ETHYL BENZENES HAVING THE GENERAL FORMULA: 